knockouts are metal or plastic tabs that the factory puts in trucks so that it holds the alignment that was set at the factory. when you get your first alignment done at a shop they usually have to remove these tabs hence why they are called knock outs. because they have to knock them out to adjust the camber and caster.

Once an alignment has been done, do they replace the tabs? Can an alignment be done on only one side of a vehicle? Apparently one side of the vehicle has had "alignment" work on it so the "knock out" is gone. I'm just trying to see if information the mechanic is true as I find it hard to trust them when it doesn't make sense.